Solar energy systems are gaining wide acceptance as a supplemental or primary source of electrical power in industry and residential applications. The efficiencies of the thin film photovoltaic (PV) modules used in solar energy systems are reaching a level wherein large scale production is becoming economically feasible in terms of cost per watt of power generated. This is particularly true for cadmium telluride (CdTe) based PV modules. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap (1.2 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (e.g., cloudy) conditions as compared to other conventional materials.
However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power depends on the ability to produce efficient PV modules on a large production scale in a cost effective manner. The ability to process relatively large surface area substrates in a vapor deposition system with minimal interruptions is a crucial consideration in this regard.
All vapor deposition systems, such as CSS (Closed System Sublimation) systems, inevitably require shut-down for scheduled maintenance, repairs, and other procedures. However, the systems must be cooled from extremely high operating temperatures (in excess of 500° C.) and very low pressures (mTorr range) prior to any such procedures being performed. For example, temperature must be reduced at a controlled rate to below about 400° C. before any of the graphite components in the system are exposed to oxygen, or such components may ignite.
The cool-down of conventional vapor deposition systems typically involves backfilling the vacuum chamber portion of the system with an inert gas, such as nitrogen. The gas provides a conductive medium and the system is allowed to cool by simple convection. This process is disadvantageous in that it takes an inordinately long time for the system components to cool. Down-times of about 4 hours for cooling conventional systems from operational temperatures and pressure to a temperature suitable for manual handling of the components are typical. The system is not producing PV modules during this time and, thus, down-time directly attributable to the cool-down process can add considerably to the overall manufacturing costs.
Vacuum furnaces are widely used in the metal fabrication industry for heating materials, typically metals, to very high temperatures (1100° C. to 1500° C.) to carry out processes such as brazing, sintering, and heat treatment with good consistency and low contamination. Reference is made, for example, to the vacuum furnaces produced by G-M Enterprises of Corona, Calif., USA. Vacuum furnaces typically utilize a quench system to rapidly cool the metal work pieces after the desired process in complete. The quench system recirculates a pressurized inert gas, typically argon, through a heat exchanger and to nozzles in the furnace directed towards the work pieces until a desired temperature of the work pieces is reached, at which time the work piece is removed from the furnace. Reference is made, for example, to U.S. Pat. No. 5,267,257. These quenching systems are not, however, suited for cool-down of an entire vapor deposition system.
Accordingly, there exists an ongoing need in the PV module industry for an improved system and method for efficient cool-down of a vapor deposition system.